The present invention relates to optical technology, and more particularly to a method and system for providing an in-line optical circulator.
Conventional optical circulators are used for many purposes. For example, conventional optical circulators may be employed in systems transmitting optical signals in order to transmit optical signals in a particular direction. In a three port optical circulator, an optical signal input at the first port will be transmitted to the second port. An optical signal input at the second port will be transmitted to the third port. However, optical signals will not be transmitted in the reverse direction. For example, an optical signal input at the second port will not be transmitted to the first port. Optical circulators can also come in a variety of configurations. One desirable configuration is an in-line optical circulator in which the first and third ports are adjacent, while the second port is at the opposing side of the system.
One prior art optical circulator is described in U.S. Pat. No. 5,909,310 by Li, et al and shown in FIG. 1A. This conventional optical in-line circulator 10 includes a first port 12, a second port 14 and a third port 16. The conventional optical in-line circulator 10 also includes a first collimator 18, a first birefringent crystal 20, a first pair of half wave plates 22A and 22B, a first Faraday rotator 24, a conventional Wollaston prism 26, a second birefringent crystal 28, a second pair of half wave plates 30A and 30B, a second Faraday rotator 30, a third birefringent crystal 34, a second collimator 36 and the fiber for the second port 14.
This conventional optical in-line circulator suffers from two disadvantages. First, the half wave plates 22A and 22D in the first pair of wave plates need to be aligned to each other. Similarly, the wave plates 30A and 30B in the second pair of wave plates also need to be aligned to each other. They are difficult to aligned respectively to each other in the manufacture process. Therefore, the alignment angular tolerance on the wave plates 22A, 22B and 30A, 30B are relatively high, which yields a lower isolation. Second, the Wollaston prism 26 is expensive and relatively more complicated to manufacture since it is composed of two wedges 26A and 26B with their optical axis parallel and perpendicular to their side direction, as shown in FIG. 1B. These two wedges 26A and 26B has to be separately manufactured and polished, then brought together to form the Wollaston prism 26. As a result, it make the manufacture process more complex and higher the cost.
U.S. Pat. No. 6,049,426 by Xie et al. (xe2x80x9cXiexe2x80x9d) describes another conventional in-line optical circulator. FIG. 2 depicts a conventional in-line optical circulator 50 in accordance with the teachings of Xie. It does not utilize any half wave plates, also eliminates one birefringent crystal, but uses an additional Wollaston prism 52 having wedges 52A and 52B. One of ordinary skill in the art will readily realize that the conventional in-line optical circulator 50 is relatively difficult to manufacture with higher cost. The optical circulator 50 suffers from two drawbacks. First, the optical circulator 50 uses two Wollaston prisms 26xe2x80x2 and 52. The cost is thus increased by the additional number of Wollaston prism. Second, since the beam deflection angular tolerance introduced by Wollaston prisms is accumulated with the number of Wollaston prisms used, the beam deflection angular tolerance introduced by Wollaston prisms 26xe2x80x2 and 52 in circulator 50 is doubled compared with the circulator with only one Wollaston prism, making optical alignment and, therefore, manufacture more difficult and complex.
Accordingly, what is needed is a system and method for providing an optical circulator that is simpler to manufacture with a lower cost. The present invention addresses such a need.
The present invention provides a method and system for providing an optical circulator. The optical circulator comprises a first port, a second port, a third port and means for establishing a first optical path and a second optical path, the second port is opposite to the first port, while the third port is adjacent to the first port. The first optical path is from the first port to the second port, while the second optical path from the second port to the third port. The optical path establishing means include a first and a second half wave plate, a first and a second rotator pair, and a polarization beam deflector. The first rotator pair is between the first port and the first half wave plate. The second rotator pair is between the second port and the second half wave plate. The polarization beam deflector is for altering a direction of the first optical path and the second optical path. The polarization beam deflector is located between the first rotator pair and the first half wave plate. Thus, when an optical signal is input at the first port, the optical signal travels along the first optical path to the second port. When the optical signal is input to the second port, the optical signal travels along the second optical path to the third port.
According to the system and method disclosed herein, the present invention provides an in-line optical circulator which can be more easily and manufactured with lower cost than conventional in-line optical circulators.